Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems related to seismic exploration and, more particularly, to mechanisms and techniques for providing a variable-depth seismic source that can achieve strong low-frequency energy, smooth spectrum, and a reduced number of high-frequency ghost notches.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the structures under the seafloor is an ongoing process.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The streamers may be disposed to have other spatial arrangements than horizontally, e.g., variable-depth arrangement. The vessel 10 also tows a seismic source array 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downward, toward the seafloor 20, and penetrates the seafloor until, eventually, a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on streamer 12. Based on this data, an image of the subsurface is generated.
In an effort to improve the resolution of the subsurface's image, an innovative solution (BroadSeis system of CGGVeritas, Massy, France) has been implemented based on broadband seismic data. The BroadSeis system may use Sentinel streamers (produced by Sercel, Nantes, France) with low noise characteristics and the ability to deploy the streamers in configurations allowing the recording of an extra octave or more of low frequencies. The streamers are designed to record seismic data while being towed at greater depths and are quieter than other streamers. Thus, the receivers of these streamers need a marine broadband source array.
A marine broadband source array may include one or more sub-arrays (usually three sub-arrays), and each sub-array may include plural source points (e.g., an airgun) provided along an X direction as shown in FIG. 2. FIG. 2 shows a single sub-array 17 having three source points 34. Such a source sub-array 17 includes a float 30 that may be connected to a vessel (not shown) via a connection 32. The float 30 is configured to float at the surface of the water or near the surface of the water and to support the plural source points 34. The source point may be not only an air gun but any other source known in the art. Source points 34 are suspended with appropriate cables 36 from the float 30 and also might be connected to each other by cables 38. An umbilical cable 40 may link one or more of the source points 34 to the vessel for providing a mechanical connection, and also electrical, pneumatic and/or communication cables. Source points 34 are typically provided at a same depth from a surface of the water.
Some disadvantages of such a source array are weak low-frequency energy, non-smooth spectrum, and the presence of high-frequency ghost notches. An alternate source array is discussed in WO 2009/005939, the entire content of which is incorporated herein by reference. This reference discloses using plural floats 40 floating at the surface 42 of the water as shown in FIG. 3. There are sub-arrays that include individual sources 44 provided at a first depth z1 and sub-arrays that include individual sources 46 provided at a second depth z2, larger than z1. However, such a configuration is still affected by the above-discussed disadvantages. Further, each float provides floating support for source elements situated at a same depth.
A source array that has better characteristics than the existing source arrays is disclosed in patent application Ser. No. 13/468,589, filed on May 10, 2012, and assigned to the same assignee as the present application, the entire disclosure of which is incorporated herein by reference. This source array is illustrated in FIG. 4 as source array 50. The source array 50 may include three different sub-arrays 60a-c, each sub-array having a corresponding float 52a-c, respectively. From each float a plurality of source points 64 is suspended. However, different from the existing sources, it is noted that source points 64 are suspended, from the same float, at two different depths, and the configuration of the source points attached to one float may be different from the configuration of the source points attached to another float. For example, FIG. 4 shows that the sub-array 60a has the higher depth source point behind the shallow source points along the direction Y while the sub-array 60c has the higher depth source point between the shallow source points along the Y direction.
However, even this improved source seems to have some limitations; for example, a natural mid-frequency “suck-out” about 10 dB in depth in its energy spectrum and lack of strong energy in the 80 to 120 Hz frequency interval.
Thus, it is desired to produce a new source array that overcomes these problems and achieves strong low-frequency energy, a smooth spectrum, and a reduced number of high-frequency ghost notches. Ghost notches occur when upwardly-travelling seismic energy is reflected or scattered downward at the sea surface. The ghost reflections are also detected by the seismic receivers and generate notches in the recorded data.